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In the following documentation:
http://docs.mongodb.org/manual/core/sharding-chunk-migration/#jumbo-chunks
we have:
By default, each document operation during chunk migration propagates to at least one secondary before the balancer proceeds with the next document, which is equivalent to a write concern of { w: 1 }. You can set the writeConcern option on the balancer configuration to set different write concern semantics.
I believe it should be { w:2 }.
This page seems to have it right: http://docs.mongodb.org/manual/tutorial/configure-sharded-cluster-balancer/
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